(1) Field of the Invention
This invention relates to a program circuit for permanently storing data in a programmable read only memory (PROM) and, more particularly, to an improvement of a program circuit for a junction-shorting or fuse-blown type PROM.
(2) Description of the Prior Art
As widely known in the prior art, a junction-shorting type PROM includes a plurality of memory cells. Each memory cell is connected between a bit line and a word line. Each memory cell usually consists of two diodes connected in series and in reverse direction to each other. Data is stored in a memory cell by electrically shorting a junction in the two diodes of the cell. Instead of each memory cell consisting of two diodes, each memory cell may be a transistor having a base open structure. This transistor is electrically equivalent to the above-mentioned two diodes. In such a base open transistor, the junction between the emitter and the base is used as the junction to be shorted, and the junction between the base and the collector is used as a diode for isolation against reverse current flow.
In order to program such a PROM, a program circuit is generally used. The known program circuit includes at least one switching circuit whose program input receives a program current, whose control inputs are connected to outputs of a decoder for decoding bit addressing signals which are received, and whose outputs are connected to bit lines. When bit addressing signals are applied to the inputs of the decoder, each switching circuit causes the program current to be conducted through one of the bit lines in response to one output of the decoder. Then, when a word line is selected and its potential is lowered, junction shorting is effectecd in the memory cell connected between the specified bit line and word line. In this way, programming of the PROM is performed.
However, the program circuits of the prior art involve problems. That is, as hereinafter described in detail, since the programming current is conducted not only through the selected bit line but, also, through other circuit elements, the programming current is wasted in these circuit elements. In addition, because the characteristics of these circuit elements are not uniform, the programming current conducted through the selected bit line is not constant. This often results in an insufficient amount of current to effect junction shorting. If insufficient current is conducted through a selected bit line, the selected memory cell is subjected to a current which is relatively large, but which is not enough to effect junction shorting, for a relatively long time. The memory cell, when subjected to a relatively large current for a relatively long time, becomes overheated and, as a result, inoperative.
The above mentioned problems in the prior art have recently become more serious because of the necessity to lower programming power, in accordance with the trend toward miniaturization of each memory cell and the accompanying shallow junctions due to high integration of PROMs.
The fuse-blown type PROM of the prior art, in which information is stored by blowing a fuse connected between a bit line and a word line, also involves the above mentioned problems.